What is a merchant account?

Every payment made with a credit card involves the transfer of funds
to a merchant account, which a merchant (that is you) holds directly
with a bank. The merchant has full responsibility for the
transactions that occur with their account, and each bank has its
own terms of service to which account‐holders must adhere. If you
want to accept credit card payments through your Shopify store, you
will need either a merchant account of your own or the services of a
third‐party payment processor like PayPal.